1. Field of the Invention
This invention relates to method for quality pretreating and improving coking coal (coal for making coke) quality, by heating, in the manufacture of coke for blast furnace use.
2. Description of Related Art
In order to maintain good permeability in the blast furnace, coke for blast furnace operation is required to have a predetermined strength. Therefore, high-quality coals (such as strongly-coking coals) have conventionally been used as the coal charge.
As high-quality coal resources are being depleted, it is necessary to use large quantities of low-quality coals (non- or slightly-coking) as a main source of coals for metallurgical coke-making and supply of blast furnace coke.
As coke strength depends greatly on the caking properties of coal and low-quality coals, and they are unsuited to use as metallurgical coals, many pretreatment methods to enhance the caking properties of low-quality coal have been proposed.
As rapid-heating is in particular effective for improving the properties (caking properties) of non- or slightly-coking coals, several coke manufacturing methods incorporating such rapid-heating have been proposed (such as Japanese Unexamined Patent Publication (Kokai) Nos. 07-109465, 07-118661, 07-118662, 07-126626, 07-126653, 07-126657, 08-127779, 08-209150, 08-259956 and 09-118883).
For example, Japanese Unexamined Patent Publication (Kokai) No. 08-209150 discloses a method for manufacturing blast furnace coke by carbonizing a blended coal comprising strongly-coking coal of 40 to 90 wt % and the balance comprising non- or slightly-coking coal in which the non- or slightly-coking coal is classified to fine coal not more than 0.3 mm in size and coarse coal over 0.3 mm in size after preheating to between 250 and 350° C., the fine coal is rapidly heated to a temperature range between the temperature at which the non- or slightly-coking coals begins to soften and the maximum fluidity temperature thereof at a rate of 1×103 to 1×105° C./min., the heated fine coal is then hot-briquetted under a pressure of 5 to 1000 kg/cm2 while being held in said temperature range, and the hot-briquetted fine coal is charged into a coke oven together with the strongly-coking coal and coarse non- or slightly-coking coal preheated to between 250 and 350° C.
This method, proposed by the applicant and with attention focused on the thermal properties of non- or slightly-coking coals, essentially comprises the steps of size classification after preheating, rapid-heating of the classified fine-size coal, hot-briquetting after rapid-heating, and blending and carbonizing with strongly-coking coal and the classified coarse-size coal, affords greater than ever improvement in the strength of coke prepared from low-quality coal, expands the range of metallurgical coal choice and provides greater productivity enhancement.
The applicant also proposed in Japanese Unexamined Patent Publication (Kokai) No. 09-118883 a method for manufacturing blast furnace coke comprising the steps of preheating charged coal to between 250 and 350° C., classifying the preheated coal to coarse and fine coals by a cyclone, adjusting the caking power index of the classified fine coal to under 80% by adding non- or slightly-coking coals, rapid-heating mixed coal to between 350 and 480° C. at a rate of 100 to 1000° C./sec., agglomerating the heated coal, blending the agglomerated coal with the classified coarse coal, and carbonizing the blended coal in the coke oven.
The conventionally proposed or disclosed methods to reform metallurgical coals by rapid-heating are fundamentally based on classifying coals into non- or slightly-coking coals and strongly-coking coals, classifying the non- or slightly-coking coals into fine and coarse coals, and rapid-heating each of the coals thus classified individually. Although, accordingly, this effectively improves the properties (caking properties) of coals, the operational efficiency is not very high because the methods involve many steps before charging into the coke oven.
With a view to affording greater use of non- or slightly-coking coals and expanding the range of metallurgical coal choices, the applicant also proposed in Japanese Unexamined Patent Publication (Kokai) No. 08-259956 a method for manufacturing blast furnace coke comprising the steps of rapid-heating a mixture of non- or slightly-coking coals of 10 to 30 wt % and the balance caking coal at a rate of 1×102 to 1×106° C./min., to a temperature range −100° C. to +10° C. of the temperature at which the mixed coal begins to soften and carbonizing the rapid-heated coal in the coke oven.
Though affording improvement in operational efficiency, rapid-heating of a mixture of strongly-coking coals and non- or slightly-coking coals implemented in this method requires further improvement in the stability of coke strength through the improvement of caking properties.
While many excellent proposals have been made in relation to rapid-heating to improve the caking properties of coals, more technologies to permit volume production of metallurgical coke to further enhance the operational efficiency and productivity of blast furnaces while increasing the use of non- or slightly-coking coals and, at the same time, securing the conventional or greater coke strength must be developed.